A membrane separation method is known as a gas separation method for selectively separating a specific gas from a mixture of various gases and purifying the gas. The membrane separation method is attracting attention because this method is an energy-saving technique as compared with other gas separation methods.
As materials for gas separation membranes for use in the gas separation method, organic membranes such as polyimide membranes have been proposed. However, such organic membranes have had a problem in that these membranes are poor in heat resistance and chemical resistance (for example, Patent Document 1). Meanwhile, it has been reported to use a zeolite, which has excellent heat resistance, in a separation membrane. However, zeolite membranes have poor acid resistance. In addition, since it is difficult to form the zeolite used alone into a desired shape, it has been necessary to form a membrane thereof on a porous support. Moreover, there has also been a problem in that the steps for producing the membrane are complicated and costly (for example, Patent Document 2).
In order to solve those problems, membrane separation using a carbon membrane is attracting attention. Carbon membranes show excellent gas-separating performance and are usable even in environments where heat resistance and chemical resistance are required, and are hence expected to be put to practical use. As the carbon membranes, for example, a carbon membrane obtained by applying a resin such as a phenolic resin or polyimide to the surface of a hollow porous ceramic body and carbonizing the resin in a non-oxidizing atmosphere, has been proposed (for example, Patent Documents 3 and 4). However, the steps for producing such a carbon membrane are complicated, and the carbon membrane has been expensive. A report has been made also on a hollow-fiber carbon membrane as a self-supporting carbon membrane (for example, Patent Document 5). Hollow-fiber carbon membranes can be produced at low cost by relatively simple production steps and can be made to have a large membrane area per unit volume. Therefore, these carbon membranes have an advantage in that a compact separation membrane module can be fabricated therefrom as compared with flat membranes. Recently, reports have been made on cases where the flexibility and brittleness which were problems in carbon membranes have been improved (for example, Patent Documents 6, 7 and 8).